Precise measurement of the flow rate of powder conveyed by a fluid is relevant to various technologies besides powder painting--food processing, toner manufacturing, and coal processing industries to name a few.
It is known to provide powder flow rate measuring devices between a fluidized powder storage tank and a powder conveyor device including a compressed air pump which imparts to the air-powder mixture sufficient energy to enable it to be conveyed in a transport to a device at which it is applied.
The prior art includes two main types of flow meters: (1) the so-called true mass flow meter, which responds directly to mass flow rate, and (2) the inferential mass flow meter, which commonly measures volume flow rate and fluid density separately. PERRY'S HANDBOOK OF CHEMICAL ENGINEERING. When measuring mass flow of pneumatically conveyed powder paint particles, one potential "true mass flow meter" relies on the Coriolis effect. However, Coriolis meters are inappropriate for powder painting production applications due to impact fusion. When particles are pneumatically conveyed through curved tubing, inertia of the particles causes some of them to collide with the walls of the tube. The energy of the impact results in fusion of some of the particles to the tube walls. Experience with the problem indicates that 1/2-inch tubes can become entirely blocked in less than three months.
Inferential mass flow meters are limited by problems associated with particle velocity and the volume ratio of powder to air (or concentration if powder density is constant).
Capacitance has also been used as a method for particle concentration measurement. The dielectric constant of a dilute particle-air mixture is is a function of the concentration of that mixture. If tubing is configured to be a capacitor, changes in particle-air concentration will change the dielectric constant, which in turn will change the capacitance of the capacitor. U.S. Pat. No. 3,636,763 is illustrative of such approaches. Other approaches in using capacitance technology to measure particle velocity are disclosed in U.S. Pat. Nos. 3,595,078 and 3,635,082.
It is now conventional to use the Doppler effect to measure velocity and concentration of fluids using laser and ultrasonic energy sources for the Doppler devices. Laser-based systems, however, are intrusive--they must be mounted so that the laser beam "sees" the flow. This limits their effectiveness in powder applications because powder buildup on the sensor window would blind a laser-based system and render it useless. Also, ultrasonic approaches tend to suffer from problems stemming from environmental noise. Additionally, the sensors tend to be very dependent upon solids content. Ultrasonic sensors are disclosed in U.S. Pat. No. 4,882,934.